Carbide vs HSS and Chatter for Small Mills

I came across a great thread this morning on the 1911Forum, a gathering place for Colt 1911 handgun enthusiasts with a good sub-forum on gunsmithing. The question was being asked of whether Carbide or HSS tooling would be better for reducing chatter on small mills. I responded as follows:

Carbide vs HSS for small mills is an interesting question, and chatter may not be the limiting factor.

There is a trade off in material removal rates and rigidity within the envelope your mill is capable of. For example, chiploads are generally higher for HSS than for Carbide, all other things being equal. So, for a given rpm, you can remove more with HSS. However, carbide will tolerate a much higher rpm that will offset this and overcome it if you can go to a high enough rpm.

Where rigidity is concerned, I would think about two rigidity issues. First is tool deflection, especially for smaller tools. Many machinists are surprised at just how much small tools can deflect. I have software called G-Wizard that does tool deflection calculations, and I can say that if you’re running less than a 1/2″ diameter tool, it may be a lot more than you expect. You should keep the tool deflection below 0.001″ when roughing to minimize the likelihood of chatter.

Chatter is a resonant phenomenon. Think of a tuning fork. Something is struck and it oscillates. You can either try to make a more rigid tuning fork, or reduce the “striking” that pumps the energy to keep the chatter going. When the tool flexes, it “pumps” the chatter, so reducing the flex reduces the energy available to cause chatter. Carbide is 3x more rigid than HSS, so it is harder to get it to chatter, all things being equal.

Keep tool stickout to a minimum, and you reduce the length of the tuning fork’s tines. Less chatter. Increase your tool diameter, even just a little bit, and you have a much more rigid tool. A 1/2″ HSS EM sticking out 1″ is 9.5x more rigid than a 3/8″ HSS EM sticking out 1″.

Second issue is machine rigidity, always a scarce commodity for smaller machines. Deal with this via horsepower. For every action, there is an equal and opposite reaction. The horsepower from your spindle pushes the cutter which pushes the workpiece which pushes the workholding which pushes the machine, and back around to the spindle. Do see another tuning fork coming up?

So if you have a 2 HP spindle, consider that if you run a 2 HP cut, you’re pumping the max into the tuning fork. You can back it off if chatter is a problem, or at least be aware that the closer to the limits you come, the more likely the chatter. Leave plenty when roughing for a light finish pass that pumps very little chatter. Calculate the cut’s HP vs your machine’s HP to estimate how much margin you have available.

Lastly, chatter is a resonant phenomenon (tuning forks again). Your machine and tooling has natural frequencies where it likes to resonate. Your job is to avoid exciting those frequencies. You may be able to do that not just by going slower, but also by going faster, oddly enough. Always try increasing feedrate, then increasing spindle rpm, before slowing down, to see if that gets you out of the chatter zone.

What should a small shop with a lightweight mill do RE carbide vs HSS?

Buy carbide below some size. I like carbide in 1/2″ diameter or less. The small tools suffer most from lack of rigidity. Buy HSS in larger sizes where the bigger diameter vs the relatively low HP of a small mill means you have enough rigidity. Big carbide is the most expensive anyway.

The limiting factor on small carbide is that it is brittle. Particularly for very small endmills, if you have much runout in your spindle or toolholder, you’re going to break more cutters. I prefer ER collet chucks for small tools. Be sure to measure the runout if you use import tooling. I had a bad collet one time in an otherwise good set that broke 3 cutters before I finally tracked it down.